Dealing with the curse of chytrid

Coming to terms with amphibian chytrid fungus in Australia’s High Country

Frogs are in trouble. A third of all frog species are threatened with extinction. The usual culprits of habitat loss and climate change are at work, but another more insidious threat looms. A devastating disease called chytridiomycosis has been wiping out frogs, often from pristine habitats. The disease is caused by a fungus – amphibian chytrid fungus (pronounced kit-tyrid). The fungus disrupts the skin function of infected frogs leading to cardiac arrest (heart attack).

The numbers are sobering. Since the identification of chytrid by Australian researchers in 1998, the pathogen has been documented in over 500 amphibian species, and is now found on all continents (except Antarctica). Fortunately, the pathogen is not universally deadly with some species demonstrating high resistance (though this produces some problems of its own).

However, many species are highly susceptible and the pathogen has been identified as the primary driver of decline for over 200 species of frog! It’s believed that 113 of these species are likely already extinct.

Although the origin of chytrid remains uncertain, recent evidence suggests the pathogen may have originated from Brazil and has since been distributed around the globe unwittingly by humans. The earliest record of chytrid in Australia is from a frog specimen in a museum collected in 1978, in south-eastern Queensland. From a potential introduction point of Brisbane, chytrid appears to have spread rapidly both north and south, reaching far north Queensland in the mid-1990s and Tasmania by 2004. The cooler, wetter conditions of the Great Dividing Range have proved highly suitable for the pathogen and its impacts have been severe along the entire east coast of Australia. While chytrid is now present throughout eastern Australia, luckily, it doesn’t tolerate the hot, dry conditions found in many inland regions.

Over the past three years Ben Scheele and colleagues have been focusing on the long-term impacts of chytrid on frogs of the Australian High Country – a region that is home to several frog species found nowhere else in the world. In the mid-1980s mysterious frog declines were reported from the region and, with the benefit of hindsight and retrospective museum sampling, we can now be confident that these declines were caused by the initial emergence of chytrid. In conjunction with Threatened Species Manager David Hunter from the NSW Office of Environment and Heritage Ben has been examing population trends of susceptible species and the ongoing threat posed by chytrid three decades after its emergence.

Although the impact of chytrid has been horrendous, it’s not all bad news. In the foothills of the Snowy Mountains, the whistling tree frog, a close relative of the alpine tree frog, is bouncing back. Surveys in the 1970s found that whistling tree frogs were ubiquitous on the NSW southern tablelands; if there was a pond or farm dam, they were just there. Like so many other species, whistling tree frog populations crashed in the 1980s and the species was considered rare in the Canberra region by the early 1990s.

When Ben commenced his surveys in 2011, he found that the species was present in many areas where it was absent two decades earlier. Ongoing surveys in 2012 and 2013 demonstrated that, year by year, the species is re-expanding into habitat occupied decades ago. Whilst more work remains to be done on the mechanism facilitating recovery (chytrid prevalence remains high and appears to drive high adult mortality), he found that sites that retained frogs during the cycle of population decline and recovery had high quality habitat. This highlights the potential for habitat to buffer species from novel shocks.

Building on his field research and a review of the international literature, Ben developed a framework to help guide the management of chytrid-threatened species. Within this framework, he identified two broad management approaches: 1. reducing chytrid fungus in the environment or on amphibians and 2. increasing the capacity of populations to persist despite increased mortality from disease.

Responding to the chytrid threat has appeared extremely daunting over the last 15 years. However, this research indicates there is real potential to make significant progress in the management of this terrible disease.